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Old 12-01-2011, 12:48 AM   #1
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uneven tile underlayment correction


i have a small, perhaps 1/4" depression spanning two adjacent beam slots because the beam in the middle is slightly lower. the beams 1 and 3 are level flush. so the area is about 30"x30" and the trough (bottom) is about, i estimate, 1/4" deep, the depression obviously tapering towards both sides.

since this is a bathroom and i will be using marble tile, i want to, naturally, level the dip. i will be putting 1/2" underlayment plywood over OSB subfloor, then thinset, then 1/4" hardibacker, then thinset again, then tile.

i was wondering in which layer and how to fix the dip. the more complex solution that comes to mind is to use self-leveling compound between the subfloor and plywood. the simpler solution is to simply dump more thinset in the area before setting hardibacker. but was concerned whether the dip is too deep for thinset to close and whether i should resort to the first solution nonetheless.

my two proposed solutions are illustrated in the attached exhibit.

thanks
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uneven tile underlayment correction-underlayment_depression_fix.jpg  

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Old 12-01-2011, 04:25 PM   #2
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uneven tile underlayment correction


Hi,

Thanks for the IM, I think we have spoken before here too.

Neither of those methods are the right way to do the job. When using concrete backer you should flatten the floor after, not before. Nails or screws don't work well into concrete products. So go ahead and install the boards then the self leveling compound.

Alternatively you can install the 1/2" underlayment then patch the floor, then install a membrane such as Ditra, then tiles. No concrete board.

You're right to install the 2nd sheet of plywood since it's a must with marble tiles, but what about the joists' deflection rating? I bet the joists do not meet specs for marble. Have you checked?

Jaz

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Old 12-01-2011, 08:00 PM   #3
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uneven tile underlayment correction


With 2 layers of wood, 3/4 OSB and 1/2 plywood, Schluter says you can go up to a 24" center on floor joists with Ditra or Ditra-XL. Taken from Schluter-Ditra Installation Handbook.
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Old 12-01-2011, 08:34 PM   #4
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Quote:
With 2 layers of wood, 3/4 OSB and 1/2 plywood, Schluter says you can go up to a 24" center on floor joists with Ditra or Ditra-XL. Taken from Schluter-Ditra Installation Handbook.
Yes of course. So, what is your point as it relates to this thread?

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Old 12-02-2011, 12:04 AM   #5
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uneven tile underlayment correction


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what about the joists' deflection rating? I bet the joists do not meet specs for marble. Have you checked?
the house is 100 years old and it that particular area, the original joists were severely damaged by the dumbarse previous owner who cut a 4" hole in one to enable a 3" PVC pipe to go through, which i fixed by sistering so it should be pretty solid now. but i do not know how to measure deflection. is there a device for it?
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Old 12-02-2011, 12:05 AM   #6
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uneven tile underlayment correction


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Originally Posted by plowboy View Post
With 2 layers of wood, 3/4 OSB and 1/2 plywood, Schluter says you can go up to a 24" center on floor joists with Ditra or Ditra-XL. Taken from Schluter-Ditra Installation Handbook.
i do not know what Ditra is. do enlighten me, please, on how it can benefit this scenario.

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